1. Field of the Invention
The present invention relates to a four-wheel driven vehicle provided with a front wheel driving system. One of three (3) modes may be selected. A two-wheel drive (2WD) mode in which on only rear wheels are driven, a first four-wheel drive (F4WD) mode in which the front wheels are driven together with the rear wheels at a peripheral speed substantially identical with that of the rear wheels, and second four-wheel drive (S4WD) mode in which the front wheels are driven together with the rear wheels at a peripheral speed higher than that of the rear wheels.
2. Related Art
A conventional agricultural tractor serving as a four-wheel driven vehicle, whose front and rear wheels rotate at a nearly equal peripheral speed are described in U.S. Pat. Nos. 4,669,559, 4,696,365, 4,723,623, 4,856,611, 4,862,988, and 5,293,956. Such a conventional tractor has the problem that, when hard cornering at the end of field during work such as cultivation, the rotation of its front wheels is too slow to follow the turning of the tractor, whereby the front wheels slip and damage the field. Recently, to avoid such damage to the field, a front wheel speed-up system has been employed by the four-wheel driven vehicle to increase the rotational speed of the front wheels and enable them to follow the turning of the vehicle without slip during hard cornering.
The conventional front wheel speed-up system is provided with a speedup clutch whose engagement makes the rotational speed of front wheels higher. The speed-up clutch is mechanically made to engage by the rotational force of a steering wheel when being steered beyond a predetermined angle. Alternatively, the speed-up clutch is engaged by an actuator when a sensor disposed at the steering wheel or on linkage between the steering wheel and the front wheels detects steering beyond the predetermined angle. The latter speed-up clutch consists of two hydraulic clutches; one of them adapted to engage for driving the front wheels at a nearly identical peripheral speed with that of rear wheels and the other adapted to engage for driving them at a higher peripheral speed. When each of the hydraulic clutches is operated to engage, a hydraulic pressure is applied to the clutch so as to make it engage. When it is operated to disengage, pressure oil is drained so as to release the hydraulic pressure and the biasing force of a spring makes the clutch disengage.
The mechanical speed-up clutch requires a large force for steering due to its mechanical structure. The latter speed-up clutch, which is switchable according to detection of steering angle by the sensor, has the fault that, when the electric or hydraulic system for the clutches malfunctions, both the clutches disengage and the vehicle is goes into 2WD mode, thereby lacking traction power. Also, it requires a large capacity for transmitting power into the front wheels, thereby requiring relatively large clutches which increases the space required for the clutches.